"We thought they were playing," says marine biologist Sebastian Fuhrmann of the environmental consulting firm IBL-Umweltplanung, whose photos of the killing of a young harbor seal will appear in the March 2015 issue of the Journal of Sea Research. "It looked really cute, but in just a few seconds, it was over."

The triumphant hunter of the harbor seal was, astonishingly, a gray seal. These soulful-eyed animals have long been thought to subsist on lowly creatures such as cod. But now the gray seal seems to be morphing into the most murderous killer of the southern North Sea.

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The same year, a gray seal in German waters was seen whirling a helpless harbor seal around by the neck; a half-eaten harbor seal carcass washed ashore the next day, according to the upcoming Journal of Sea Research study.

Gray seal DNA has also been found deep inside bite marks on the bodies of badly battered harbor porpoises, according to a pair of studies published in 2014 in PLOS ONE and Marine Ecology Progress Series.

Thanks to DNA analysis, scientists can trace certain porpoise injuries to gray seals. That's because the seals' handiwork often causes large areas of missing skin and blubber, as well as three to five parallel scratches on their prey's skin, according to a November 2014 study in the Proceedings of the Royal Society B.

Though scientists have ruled out a few other culprits for the mangled corpses, including other predators such as Greenland sharks, some biologists are still skeptical that gray seals are primarily responsible. (See "Slow Sharks Sneak Up on Sleeping Seals [and Eat Them]?")

For instance, biologist Dave Thompson believes that many of the harbor seals thought to be eaten by gray seals have actually been torn apart by ship propellers, and that the gray seals scavenge them after death.

"The propeller-damaged carcasses seem to be turning up wherever we look, so the problem is very widespread and still massively under-reported," said Thompson, of the University of St. Andrews in Scotland.

Even so, it's clear "we have a new top predator in the North Sea," especially for harbor porpoises in the last four years, concludes Thibaut Bouveroux of South Africa's Nelson Mandela Metropolitan University.

Another idea is that these marauders of the waves have simply returned to their old habits: The mammals have recolonized the North Sea's southern stretches after being mostly wiped out in the region due to overhunting. (Also see "How a Leopard Seal Fed Me Penguins.")

Whatever the reason, other animals should keep a wary eye out for the appealing seal with the huge, liquid eyes and clownish flippers.

"Just because they're cute doesn't make them less of a predator," says biologist Abbo van Neer of Germany's University of Veterinary Medicine Hannover. "Yes, it's bloody. Yes, it's gruesome. That is just the way nature is."